This project is concerned with the mechanisms whereby chemical carcinogens and UV radiation induce chromosomal aberrations in human fibroblast and how cell cycle checkpoint controls protect against induced clastogenesis. Structural alterations in chromosomes typify cancer and human genetic disease syndromes which include chromosomal fragility as part of their phenotypes. Environmental carcinogens such as polynuclear aromatic hydrocarbons, nitrosamines and ultraviolet radiation induce chromosome aberrations (CA's) in an S-phase-dependent process. Carcinogen-DNA adducts and UV-induced pyrimidine dimers do not produce CA's directly; the primary lesion in DNA is converted to a DNA double strand break in a process involving DNA replication. Replication-dependent DNA double strand breaks (dsb), if left unrepaired through G2, are seen as chromatid breaks when cells enter mitosis. DNA repair pathways reduce the yields of induced CA's by removing the primary DNA lesions before DNA replication and by rejoining the replication-dependent DNA dsb before mitosis. Pathways of cell cycle checkpoint control impose delays in progression through the cell cycle to provide more time for excision repair and rejoining of breaks. Ataxia telangiectasia (AT) cells, which display chromosomal instability and UV radiation hypersensitivity, also exhibit a defect in checkpoint control and fail to impose proper cell cycle delays in response to DNA damage. The contribution of the cycle delay responses to protection against clastogenesis will be determined by comparing normal human and AT fibroblasts strains. If AT cells are unable to induce cell cycle delays in response to DNA damage and display elevated yields of carcinogen induced CA's, a protective role for cell cycle delays in chemical and radiation clastogenesis would be suggested. Cell division control (CDC) kinase activities will be examined during carcinogen-induced delays to establish the biochemical signatures of cycle-delay responses in human cells. The DNA dsb that develop during replication of damaged DNA will be quantified by neutral elution chromatography to determine whether normal and AT cells develop similar levels of pre-clastogenic damage carcinogen exposure. Fibroblasts from patients with Li-Fraumeni familial cancer syndrome will also be tested for loss of specific elements of checkpoint control in response to DNA damage. Immortal LFS fibroblasts which have lost the wild- type p53 allele also have lost the G1 checkpoint control function. Demonstration of enhanced clastogenesis in these immortal LFS would lend further support to a hypothesis that checkpoint controls protect against carcinogen-induced genetic damage. This project will establish whether chemical carcinogens and UV induce chromosome aberrations in human fibroblasts by an S-phase dependent mechanism and test whether cell cycle checkpoint controls protect against cell cycle-dependent clastogenesis.